1. Field of the Invention
This invention relates to an image forming apparatus, and, more specifically, to an image forming apparatus which is capable of duplex copying, i.e. forming images on both surfaces of a sheet. This invention is also applicable to an image forming apparatus which is capable of overlay copying, i.e. forming images plural times on the same surface of a sheet.
2. Description of the Prior Art
Recently image forming apparatus providing a variety of functions, for example, the functions capable of duplex copying and overlay copying have been proposed and come into commercial acceptance. In order to achieve duplex copying and/or overlay copying, the image forming apparatus comprises a receiving unit for receiving a sheet on which a first image is formed, and the sheet is re-fed out of the receiving unit to form a second image thereon.
In a conventional duplex and/or overlay image forming apparatus, the receiving unit comprises an inlet for receiving the sheet, a tray member for supporting the received sheet, an aligning device for aligning the sheet on the tray member, and an outlet through which the received sheet is re-fed. The aligning device comprises a pair of standing members which stand upright and extend to a level lower than the inlet. The standing members are positioned in spaced relationship with each other in a widthwise direction (in a direction perpendicular to a feed direction), and slidable toward each other by a driving means. Before the sheet is received in the tray member, the standing members are set at a first position where they are positioned in spaced relationship with each other a predetermined distance much wider than the width of the sheet so that the tray member can receive the sheet between the standing members certainly. Then the sheet is received in the tray, the standing members are slide by the driving means to a second position where they are in contact with the lateral sides of the sheet, and the sheet is aligned.
The conventional apparatus, however, has the following problems to be solved.
Firstly, mainly due to the lowness of the top of the standing members, the sheet turns aside during receiving in the tray member from the inlet. Therefore the standing members are positioned where they are spaced from each other a predetermined distance much wider than the width of the sheet so that the tray member can receive the sheet between the standing members certainly. When the tray member receives the sheet, the standing members slide to a second position where they are in contact with the sheet. Moreover, the standing members have to be slid to align the sheets every time each sheet is received. However, it takes much time to align the sheets so the image forming speed cannot be so fast.
Secondly, the image forming speed may be fast, when a motor having much torque is adopted for sliding the standing members fast. Alternatively, a particular structure may be adopted for the purpose. However, such motor or such a particular structure is large in size, relatively complex in structure, and the cost is relatively high.